The present invention relates to surface emitting lasers, and more particularly, to means for phase locking and combining the outputs of a pair of such lasers.
Surface emitting lasers have an advantage over edge emitting lasers in that since their light emitting surface (the area of a grating) is larger, the power density is lower, and therefore, more power can be generated without destructive heating effects. Further, the active section of a surface emitting laser can be made longer than that of a Fabry-Perot (FP) cavity laser for more gain without spurious frequency generation due to the use of the grating. For still higher power outputs surface emitting lasers can have their outputs combined using an optical waveguide and a grating or distributed Bragg reflector (DBR) such as shown in FIG. 1 of the article "Dynamically Stable 0.degree. Phase Mode Operation Of A Grating-surface-emitting Diode-laser Array," by N. W. Carslon et al., Optics Letters, Volume 13, No. 4, April 1988, pp. 312-314. However, in such devices, due to losses in the waveguide, the phase locking of the numerous lasers may not be sufficient to prevent spurious frequency generation as well as an incoherent light beam with a broad main beamwidth and high amplitude and broad beamwidth sidelobes. It is known from P. Zory et al., "Grating-Coupled Double-Heterostructure AlGaAs Diode Lasers," IEEE Journal Of Quantum Electronics, Volume QE-11, No. 7, July 1975, pp. 451-457, to longitudinally align two lasers. However, the power is limited to that of a single pair of lasers.
It is, therefore, desirable to have a high power output from surface emitting lasers with good phase locking and a coherent output light beam.